Add ticket length before buffering DTLS message

In ssl3_send_new_session_ticket the message to be sent is constructed. We
skip adding the length of the session ticket initially, then call
ssl_set_handshake_header, and finally go back and add in the length of the
ticket. Unfortunately, in DTLS, ssl_set_handshake_header also has the side
effect of buffering the message for subsequent retransmission if required.
By adding the ticket length after the call to ssl_set_handshake_header the
message that is buffered is incomplete, causing an invalid message to be
sent on retransmission.

Reviewed-by: Richard Levitte <levitte@openssl.org>